1. Field of the Invention
The present invention relates to a lens driving device that moves a lens along an optical path for use in a variable magnification optical system in a copying machine, image reading device and the like.
2. Description of the Related Art
Copying machines having variable magnification copying functions require that a projection lens be movable along the optical path. In such cases two types of lenses are typically used, a fixed-focus lens and a zoom lens.
Conventional lens driving devices that move a fixed-focus lens typically have a rope attached to a lens mounting bed and cause said rope to travel by rotating a pulley. Lens driving devices using a zoom lens, in addition to the aforesaid moving mechanism comprising a rope and pulley, provide a second mechanism that changes the focal distance of the lens itself in correspondence with the movement of said lens.
For example, Japanese Utility Model Application No. 59-46348 discloses the previously described lens moving mechanism comprising a rope and a pulley and an additional cam mechanism which rotates a zoom ring in connection with the movement of the lens.
In the previously described conventional lens driving device, however, slipping may readily occur at the winding portion of the rope and pulley. When a slip occurs, the rope travel not only becomes imprecise, but a loss of drive torque is also induced so that drive efficiency is impaired.
Thus, the rope must have excess winding on the pulley so as to relieve said slip. However, such an arrangement has the disadvantage of complicating assembly. Further, the point of attachment of the rope to the lens mounting bed also has disadvantages inasmuch as it produces a deleterious effect on the assembly operation.
On the other hand, when a zoom lens is used as the lens, a mechanism is required to operate the zoom ring, thereby having the disadvantage of enlarging the size of the device.